This invention relates to power oil and gas burners of the type employing centrifugal blowers, and more particularly to air flow control devices for use with blower squirrel cages to improve the air outputs of the blowers.
Over the past two decades there has been a strong movement in connection with oil and gas fired boilers and furnaces, toward higher efficiencies, particularly higher annual efficiencies. Consequently the boiler or furnace units have become more compact and their heat exchangers now are more airtight and more resistant to the flow of flue gasses through them.
Such boiler and furnace developments have put a much greater demand on this type of power oil or gas burner to provide improved blower pressures and stability so that the flame would not pulsate upon ignition or shutdown.
Heretofore when using a wide, close-fitting blower wheel or rotor in a power oil or gas burner housing the air delivery was not sufficiently uniform to give reliable, pulsation-free stable combustion with many of the latest boilers or furnaces. As at present understood, the reason was that the blades of the rotors operated at or near their "stall" speed or condition, functioning much like the air foil structure of an airplane wing when the air speed is too low.
It was found that, by using a narrower wheel or rotor (typically about 2/3 as wide as the housing space) and by locating it away from the air inlet side or ring of the housing, the rotor blades never handled volumes of air that were low enough to cause a stall condition. This is because, even at a no-flow condition of the burner, the rotor is recirculating enough air to prevent air stall.
Even so, such prior arrangments did not fully approach the maximum high pressures that could be obtained, because much of the kinetic energy imparted to the air was dissipated in the recirculation process. By using a larger diameter rotor than that intended for the original housing design, the pressures obtained had been increased slightly but then the resultant whistle (siren effect) became intolerable. It was found that by adding a scroll extension which had a short straight portion, such whistle could be reduced, and that by adding a side baffle to the scroll the pressure could be increased by about 30% over previous standard designs.
However, these prior improvements all left something to be desired because burner performance was still thought to be not maximized, considering the power available, the energy imparted to the rotor and the noise levels determined to be tolerable or acceptable. It was believed that the problem of inefficient air flow still existed, as well as that of noise reduction.